(a) Technical Field
The present invention relates to a manifold insert having distribution guides and a fuel cell stack comprising the same. More particularly, it relates to a manifold insert having a plurality of distribution guides which can reduce output voltage deviation and prevent flow instability by stably distributing the flow of fluid supplied to a fuel cell stack, and a fuel cell stack comprising the same.
(b) Background Art
A fuel cell is an electricity generation system that electrochemically converts chemical energy directly into electrical energy in a fuel cell stack instead of converting chemical energy of fuel into heat by combustion. The fuel cell can be applied to the electric power supply of small-sized electrical and electronic devices, for example portable devices, as well as industrial and household appliances and vehicles.
A typical fuel cell system mounted in a vehicle comprises a fuel cell stack for generating electricity by an electrochemical reaction; a hydrogen supply system for supplying hydrogen as a fuel to the fuel cell stack; an oxygen (air) supply system for supplying oxygen-containing air as an oxidant required for the electrochemical reaction in the fuel cell stack; a thermal management system (TMS) for removing reaction heat from the fuel cell stack to the outside of the fuel cell system, controlling operation temperature of the fuel cell stack, and performing water management functions; and a system controller for controlling overall operation of the fuel cell system. The fuel cell system generates electricity by the electrochemical reaction of hydrogen and oxygen-containing air, and generates heat and water as by-products.
One of the most attractive fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC). The fuel cell stack included in the PEMFC comprises a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate. The MEA includes a polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is disposed on each of both sides of the polymer electrolyte membrane. The GDL functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member functions to provide an appropriate bonding pressure. The bipolar plate functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
The fuel cell stack is formed of a plurality of unit cells, each unit cell including an anode, a cathode and an electrolyte (electrolyte membrane). Hydrogen is supplied to the anode (“fuel electrode” or “oxidation electrode”) and oxygen-containing air is supplied to the cathode (“air electrode”, “oxygen electrode” or “reduction electrode”).
In a fuel cell vehicle having the above-described configuration, the MEA, the GDL, and the bipolar plate are sequentially and repeatedly stacked to form the fuel cell stack. Moreover, a manifold is integrally formed on the bipolar plate in the stacking direction of the unit cells. The manifold forms flow fields in the form of columns such that hydrogen, oxygen (air), and coolant flow through the flow fields.
Meanwhile, in the fuel cell vehicle, the rate of fuel consumption for each load is maintained constant to optimize the amount of hydrogen and oxygen consumed, which is defined as the stoichiometric ratio (SR). For example, the amount of hydrogen required to generate a current of 300 A is 231 SLPM (standard liters per minute), which is calculated by the stoichiometric ratio (SR).
To remove impurities, such as condensed water, and increase the potential in an actual system, the fuel is supplied in a stoichiometric ratio of 1.1 to 2.0 by supplying surplus hydrogen gas, which is the same as a gas containing an oxidant.
However, if a fuel cell stack with a higher volume power density (kW/L) is used to improve the power performance of the vehicle, the number of unit cells stacked in the fuel cell stack is increased. This increase causes non-uniformity of the flow rate of gas supplied to channels of the unit cells via the manifold for distributing the gas.
As the number of unit cells in the fuel cell stack increases, the size of the manifold is also increased such that the manifold can accommodate the flow of gas required for high power generation. Therefore, if the number of unit cells exceeds a predetermined range, the flow rate range for voltage stabilization is restricted or the stoichiometric ratio should be maintained at a high level, which causes deterioration of fuel efficiency. Therefore, it is necessary to provide an optimum manifold which can ensure a uniform flow of gas supplied to the unit cells without deteriorating the fuel efficiency.
FIGS. 1 and 2 show the distribution of the flow of fluid supplied to a fuel cell stack during partial load operation (at low flow rate, FIG. 1) and during full load operation (at high flow rate, FIG. 2) with respect to the cell number.
As shown in FIG. 1, when the flow rate of gas supplied to the fuel cell stack is low during partial load operation of the vehicle, the difference in the flow rate between the positions of the unit cells is not significant. However, as shown in FIG. 2, during a rapid change in load according to a driver's request, such as sudden acceleration, the flow rates according to the positions of the unit cells are not uniform, thus increasing the output voltage deviation.
In the unit cells adjacent to an inlet (e.g., first to third cells), there are regions where the flow rate is insufficient due to flow instability caused when a change in flow rate occurs due to a rapid change in load, compared to the unit cells spaced a predetermined distance. While the cross-sectional area of the manifold is constant, the amount of gas that can reach the unit cells spaced a predetermined distance is reduced. Therefore, an excessive amount of gas is supplied to the unit cells, but there are regions where the flow rate is insufficient due to a partial reduction in gas pressure, which results in a reduction in power generation. As a result, this leads to a limitation in power generation and, if this non-uniformity of the flow rate of gas supplied to the unit cells is out of the control range, it may develop into a failure in start-up and a shutdown of the vehicle.
In the past, in an attempt to solve these problems, a bipolar plate, in which a manifold is provided in the form of an internal manifold to minimize the design change of the existing bipolar plate and uniformly distribute the flow, has been proposed.
There are also various other techniques that have been proposed for addressing these problems, such as a technique for providing a uniform distribution of gas by changing the order of manifolds according to their sizes or changing the arrangement of bipolar plates, a technique for addressing the non-uniform flow rate of gas supplied to unit cells by stabilizing the flow rate before the gas is supplied to channels, a technique for stabilizing the flow rate by changing the shape and form of a separation window to separate the flow according to the size of a fuel cell stack and the flow velocity through a change in the stacking order of bipolar plates, etc.
For example, Korean Patent No. 0579308 (hereinafter “Patent Literature 1”) describes a system for stabilizing flow distribution which comprises a flow distribution structure with a curved surface, a separation window for separating the flow of fluid supplied to manifolds at a predetermined ratio, a dummy cell, and a flow obstacle, in which the separation window is formed during manufacturing of a bipolar plate and comprises a plurality of stepped portions formed of a porous material.
Korean Patent Application Publication No. 10-2008-0076206 (hereinafter “Patent Literature 2”) describes a manifold of a fuel cell stack which has a structure in which the cross-sectional area of an inlet manifold for supplying hydrogen and air to unit cells of the fuel cell stack is reduced as it goes from an inlet to an end portion, and an outlet manifold that increases as it goes from a start portion to an outlet.
Korean Patent No. 0637506 (hereinafter “Patent Literature 3”) describes a fuel cell system which comprise a manifold whose cross-sectional area is reduced toward the center such that fuel and oxygen in an amount corresponding to a difference in temperature at the position of each power generation unit are supplied to a corresponding unit cell.
Korean Patent No. 0831462 (hereinafter “Patent Literature 4”) describes a fuel cell stack which comprises a dual manifold including a flow stabilization manifold, and which is formed by stacking bipolar plates to uniformly distribute the flow of fluid to a plurality of unit cells.
Japanese Patent Application Publication No. 2006-147456 (hereinafter “Patent Literature 5”) describes a gas supply manifold of a fuel cell stack which comprises a flow stabilization unit (buffer) added to an external supply tube.
However, the above-described conventional techniques disclosed in Patent Literatures 1 to 5 have the following problems.
First, according to Patent Literature 1, the separation window provided for the flow stabilization should be formed in the manifold during manufacturing of the bipolar plate, and thus it is difficult to mass produce the bipolar plates used in multiple unit cells (e.g. 400 unit cells) per fuel cell stack. Moreover, a predetermined length of the separation window is formed in a straight line and the plurality of stepped portions is formed thereafter, but the flow instability is not completely solved. Therefore, the porous flow obstacle in the form of a net and the dummy cell are further provided. However, the shape of the porous flow obstacle is complex, and thus its production is difficult. Moreover, the unstable flow region is filled with the dummy cell, which causes an unnecessary increase in volume and cost.
In Patent Literature 2, the arrangement of the bipolar plates is controlled to gradually increase or decrease the cross-sectional area of the manifolds. However, the production of the fuel cell stack is thereby complicated, reducing the productivity.
Moreover, in Patent Literature 3, when the cross-sectional area of the manifold is reduced toward the center to control the amount of heat generated, the flow is unstable, thereby causing voltage instability and damage to the unit cells.
Furthermore, in Patent Literature 4, when the flow stabilization manifold is formed in the bipolar plate, the production of the bipolar plate is complicated and the production cost increases.
In addition, in Patent Literature 5, when the buffer is provided on the outside of the fuel cell stack, the unused space of the system increases, and thus it is difficult to form the fuel cell stack in a compact size. Moreover, it is difficult to immediately satisfy the driver's request such as sudden acceleration using this system.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.